But for casual speech, you would use the following rule: If it is 1 kg, or 1 L, or one of anything at all, then put "was added". If it is any other number, put "were added".

When to add was and were?

Was is used in the first person singular (I) and the third person singular (he, she, it). Were is used in the second person singular and plural (you, your, yours) and first and third person plural (we, they). I was driving to the park. You were drinking some water.

Were included or was included?

I think both are correct. "You are not included" -- "included" is an adjective; "you were not included" -- "were included" is a verb. The meaning is essentially the same.